Preheat oven to 400 degrees.

Wash gourmet potatoes if not pre-washed. Do not peel. Toss potatoes in a bowl with 2 Tbsps of the olive oil and ¾ teaspoon salt. Pour water to depth of ¼ inch in a baking dish large enough to hold the potatoes in one layer. Cover dish with foil and cook until just tender. Check after 25-30 minutes. Do not overcook the potatoes at this point.

Transfer potatoes to a plate and allow them to cool about 15 minutes. Refrigerate for at least one hour.

While the potatoes are cooling in the fridge, peel and cut the russet potatoes into 2 or 3 pieces each. Cover the potatoes by 1 " in a large saucepan. Bring to a simmer over high heat. When the water is simmering add 3 TBSPs of salt to the water. Cover and allow to simmer until they are tender. Check after 30 minutes, although it may take upto 40 minutes. When tender, remove the potatoes from the heat and allow them to sit in the cooking water another 5-10 minutes.

Drain the potatoes and pass through a potato ricer.

Add 1 cup of the Grilled Onion Cream Sauce to the riced potatoes and mix. Add the egg yolk, the remaining ¾ teaspoon salt and the pepper. Mix them well with a fork. Set them aside but do not refrigerate. The filling should be quite thick and hold its shape. If it seems not stiff then add another egg yolk and if it is too stiff to pipe add more cream sauce.

Remove gourmet potatoes from fridge. Line a baking sheet with parchment paper. Cut larger potatoes in half if necessary. Trim the base of each potato just enough so they will sit flat on the baking sheet. Trim the top off the whole potatoes. Scoop out potato flesh from halves or whole potatoes carefully with a melon ball scooper or a small spoon. Be careful to leave about a ¼ " of flesh around the outside of the potato. Reserve the flesh for another use if desired.

Return the oven to 400 degrees.

Brush the insides of the potato shells with the remaining 1 tablespoon olive oil and sprinkle the insides with the remaining ½ teaspoon salt. Put the Filling mixture into a piping bag and fill the potato shells so the filling sits about 1" above the rim of the potato shells.

Bake in the center of the oven 15-20 minutes until they are gold brown and hot through. Remove from oven, sprinkle them with the finishing sea salt.

Heat the remaining Grilled Onion Cream Sauce in the microwave gently. To serve pour a few Tbsps of the sauce on the plate and position potatoes on top of the sauce.